1. Field of the Invention
The present invention relates to an X-ray image sensing device, and more particularly to an X-ray image sensing device fabricated utilizing a Thin Film Transistor (TFT) array process.
2. Description of Related Art
An X-ray detection method for medical diagnosis which has been widely used is such that an X-ray detecting film is used to produce a photograph and some predetermined printing procedure is required to obtain the result.
However, digital X-ray sensing devices (referred to hereinafter as X-ray sensing devices) employing a TFT have been developed recently due to the development of semiconductor technology. This X-ray sensing device has an advantage that a real time diagnosis can be obtained immediately after photographing because it uses a TFT as a switching element.
FIG. 1 is a schematic cross-sectional view illustrating the structure and operation of an X-ray image sensing device 100 which comprises a lower substrate 1, a thin film transistor 3, a storage capacitor 10, a pixel electrode 12, a photoconductive film 2, a protection film 20, a conductive electrode 24 and a high voltage direct current (DC) power supply 26.
Photoconductive film 2 produces internal electric signals, i.e. electron-hole pairs in proportion to the strength of external signals, such as incident electromagnetic waves or magnetic waves. That is, the photoconductive film 2 serves as a converter to detect external signals, particularly X-rays, and convert them into electric signals. Either the electrons or holes are gathered at pixel electrode 12 located beneath the photoconductive film 2, depending on a voltage (Ev) applied to the conductive electrode 24 by the high voltage DC power supply 26, and then are stored in storage capacitor 10 formed in connection with a ground line grounded externally. Charges stored in the storage capacitor 10 are transferred to the TFT 3, controlled externally, to an external image display device to form X-ray images.
In an X-ray image sensing device, to detect and convert even the weakest X-ray signal into an electric charge, it is required to decrease the trap state density for the electric charge in the photoconductive film 2 and to decrease the amount of current in non-vertical directions by applying a relatively high voltage (more than 10V/xcexcm) in the vertical direction between the conductive electrode 24 and the pixel electrode 12.
FIG. 2 is a cross-sectional view schematically illustrating a conventional X-ray image sensing device. U.S. Pat. No. 5,498,880 discloses an example of this kind of structure, wherein the pixel electrode 12 extends over the upper part of TFT 3 to prevent the trapping of electric charges at the upper part of TFT 3, induced from the electric charges produced in photoconductive film 2 by X-ray energy.
A capacitor electrode 40 made of transparent conducting material is formed on the gate insulating layer 34a on the substrate 1. A dielectric layer 34b is formed on the capacitor electrode 40 and patterned to form the ground line 42, which should extend to the adjacent pixel. The ground line 42 is formed on the exposed portion of the capacitor electrode 40. A protection layer 46 is formed on the TFT 3 to protect it from external impacts or humidity. A pixel electrode 12 is formed on the protection layer 46 contacting the dielectric layer 60 through a contact hole formed in the dielectric layer 34b. The pixel electrode 12 and the capacitor electrode 40 are the two electrodes of the storage capacitor.
The ground line 42 contacting the capacitor electrode 40 discharges the residual charges remaining in the storage capacitor.
FIG. 3a is a plan view illustrating a section of the conventional X-ray sensing device and shows a gate line 50 disposed crosswise and a data line 52 disposed lengthwise. A TFT 3 is formed as a switching element in the region where the gate line 50 and data line 52 cross and a ground line 42 is disposed parallel to the data line 52. The ground line 42 and the dielectric layers 34b are positioned on the capacitor electrode 40. The pixel electrode 12 covers the ground line 42 and the dielectric layers 34 to form the storage capacitor. The ground line 42 extends to the adjacent pixel (not shown) and contacts the capacitor electrode of the adjacent pixel.
FIG. 3b is an enlarged view of portion A of FIG. 3a, showing an electrical line short that often occurs in the conventional X-ray image sensing device. In the figure, line short portions Axe2x80x2 are seen in the ground line 42 at the boundary with the capacitor electrode 40. This occurs because a metal used for the ground line 42 is not adhered well on the capacitor electrode 40 made of transparent conducting material and the etching solution invades from both ends to inside the step difference region during the wet etching of the ground line 42.
The defect can be seen more clearly in FIG. 3c that is a cross-sectional view taken along the line IIIxe2x80x94III of FIG. 3b, showing the Axe2x80x2 part where the ground line 42 has an open portion in the step difference region of the capacitor electrode 40 and ground line 42.
The open in the ground line may cause the problem that the residual charges can not be removed efficiently, which deteriorates the quality of X-ray images.
This invention has been developed in order to address the above-described problem.
An object of this invention is to provide an X-ray image sensing device wherein the ground line open is not caused.
Another object of this invention is to provide an X-ray image sensing device having a ground line with reduced resistance.
In order to accomplish the above objects, this invention provides in one aspect, an X-ray image sensing device having a plurality of pixels in a matrix configuration, a pixel including a photoelectric conversion part to receive X-rays and generate electric charges from the X-rays; a charge storage part to accumulate the electric charges produced in the photoelectric conversion part, having a capacitor electrode, a pixel electrode and a dielectric layer located between the pixel electrode and the capacitor electrode, which extends to an adjacent pixel; a ground line for discharging residual charges in the charge storage part, positioned on the capacitor electrode to electrically contact the capacitor electrode, and extending to the adjacent pixel; and a switching part to control release of the charges stored in the charge storage part.
This invention also provides in another aspect, an X-ray image sensing device having a plurality of pixels in a matrix configuration, a pixel including a photoelectric conversion part to receive X-rays and generate electric charges from the X-rays; a charge storage part to accumulate the electric charges produced in the photoelectric conversion part, having a capacitor electrode, a pixel electrode and a dielectric layer located between the pixel electrode and the capacitor electrode; a ground line for discharging residual charges in the charge storage part, positioned on the capacitor electrode to electrically contact the capacitor electrode, and extending to an adjacent pixel; and a switching part to control release of the charges stored in the charge storage part, wherein the capacitor electrode is shaped so that an end contact line of the capacitor electrode with the ground has a length greater than a width of the ground line.
Preferably, the photoelectric conversion part is comprised of a photoconductive film and conductive electrode.
Preferably, the conductive film is preferably manufactured of a material selected from the group consisting of amorphous selenium, HgI2, PbO, CdSe, thallium bromide and cadmium sulfide.
Preferably, the ground line is preferably made of Chrome.
Preferably, the capacitor electrode is preferably made of transparent conducting material.